Magnetic disk devices have made remarkable progress, as external storage devices for computers, owing to their superior cost/performance ratio and further growth is expected. An aluminum type substrate has been used in the past as a substrate of a magnetic disk which is mounted in the magnetic disk device, but glass substrates made of chemical tempered glass and crystallized glass have gradually gained wider application because they have high impact resistance and can easily be made flat. In other words, the aluminum type substrate can easily provide a magnetic disk having excellent magnetic characteristics but involves the problem of flatness because it suffers plastic deformation during a mechanical process such as polishing. In contrast, the glass substrate can easily be made flat because it has high surface hardness and does not involve the plastic deformation described above.
When the surface of these glass substrates is polished, protrusions of various shapes, called “ski jumps”, occur in the shape of the outer peripheral edge portion owing to slight distortion, warp, polishing stress, and so forth. These protrusions make floating and moving of a magnetic head unstable and invite the occurrence of a head crash. Consequently, it is difficult to achieve a low floating height of the head and a high recording density and to enlarge a recording area.
Various studies have therefore been made to planarize the outer peripheral edge shape of the glass substrate.
For example, a method that decides the edge shape by taking a shape change, when chemical treatment is applied to the glass substrate, into consideration and provides a glass substrate, before chemical treatment, having the shape so decided (Japanese Unexamined Patent Publication (Kokai) No. 2001-167427) is possible. Another method sets a specific radial curvature (RC) on the slope of a ski jump to 50 nm or below (Japanese Unexamined Patent Publication (Kokai) No. 2003-242627). Still another method conducts polishing so that a product (Pt) of a polishing pressure (P) and a polishing time (t) falls within a certain range (Japanese Unexamined Patent Publication (Kokai) No. 2003-89459). Still another method suppresses a radius of curvature representing surface sagging of the outer peripheral edge potion at a reference length of 2 mm or 4 mm, that is arbitrarily selected, to 150 nm or below (Japanese Unexamined Patent Publication (Kokai) No. 2003-290365).
However, the inventor of this invention has found that the outer peripheral edge shape of a glass substrate for a magnetic disk involves the following problems.
To produce a magnetic disk, a base film, a magnetic recording film and a protective film are formed on a glass substrate for a magnetic disk. When any fine particles at the time of film formation of the protective film adhere to the surface of the protective film, they are removed by a burnish treatment (that brings a lapping tape into contact with the surface of the protective film by using a rubber contact roll, for example, and gently polishes the surface). In this way, the floating distance of the magnetic recording head can be reduced. However, dust is likely to fall from the tape when the tape comes into contact with the outer peripheral edge portion of the glass substrate and the resulting particles adhere to the surface of the magnetic disk, so that the magnetic disk is rejected as a rejected product in the final inspection.